Composition Of Tourmaline Research Articles

In-situ elemental and boron isotopic variations of tourmaline from the Koktokay pegmatitic rare-metal deposit, China: Insights into external contamination and the source of the granitic pegmatite

This study presents petrochemical and boron isotope compositions of tourmalines from the fertile (No. 3) and relatively barren (No. 1, 2b) pegmatite in the Koktokay pegmatitic rare-metal deposit, to better understand the influence of external contamination on pegmatite mineralization and the source of these pegmatites. There are four types of tourmalines that have been studied: (1) tourmaline in the tourmaline-mica nest of pegmatite No. 1 (Tur-N); (2) tourmalines in the comb-like structure of pegmatite No. 1 (Tur-C1), 2b (Tur-C2), and 3 (Tur-C3), respectively. Tur-N tourmaline is heterogeneous in backscattered electron (BSE) images, Tur-C1 and Tur-C2 appear to be homogenous crystals, while Tur-C3 exhibits to have a complicated zonal structure. These tourmalines are members of the alkali group tourmalines representing mostly dravite-schorl solid solutions. Tur-N contains the lowest Ca content. There is a general increase in the contents of Mg and Ca from Tur-C1 to Tur-C3, implying that the pegmatite melt was contaminated by the host rock (metagabbro) during the emplacement. The contamination's diffusive influx of Mg, Ca, and Fe components into the pegmatite melt impelled the crystallization of tourmaline during the emplacement, which subsequently triggered the deposition of rare element minerals. The external contamination implies that the closed system of melt is extremely favorable for the enrichment and mineralization of rare elements in the pegmatitic rare-metal deposit. The boron isotopes vary slightly among the four types of tourmalines. The B-isotope composition of Tur-C3 is the heaviest (–11.63 ± 0.05 ‰ to –13.16 ± 0.05 ‰), followed by Tur-C1 (–12.29 ± 0.06 ‰ to –12.93 ± 0.05 ‰) and Tur-C2 (–12.51 ± 0.05 ‰ to –13.21 ± 0.05 ‰), Tur-N contains the lowest δ11B value (–13.20 ± 0.05 ‰ to –13.76 ± 0.05 ‰). Synthesized with the previous research results, we hold that the boron isotope of tourmaline (Tur-C3) in the comb-like structure of No. 3 pegmatite might represent the isotope composition of its initial magma, indicating that the tourmaline and associated pegmatite may derive from the melting of clastic sedimentary rock from the Habahe Group forming in Mid Ordovician to Early Devonian in the Chinese Altay orogenic belt.

Formation of tourmaline in the Bakoshi-Gadanya Goldfield, Nigeria: Insights from elemental compositions and boron isotopes

Tourmaline and gold mineralization form an association in the Bakoshi-Gadanya (BAG) Goldfield on the northern West Nigerian Subshield, located in the southern Trans-Saharan Orogenic Belt of West Africa, although the cogenicity of this association remains untested. In this study, we report the results of an integrated study of the boron isotopic and major- and trace-elemental compositions of BAG tourmalines, from which we infer the nature and origin of the hydrothermal parent fluids and their role in the associated gold mineralization. Tourmalines in the BAG Goldfield are of four types: altered granite-related (Tur I), wallrock-hosted (Tur II), Gadanya Tourmalinite (Tur III) and Shanono Tourmalinite (Tur IV). The tourmalines mainly belong to the alkali group with dravite (i.e., Mg-rich) compositions. The MgFe−1 and □Al(NaR)−1 exchange vectors are the dominant substitution mechanisms for all BAG tourmalines with contributions from deprotonation AlO[R(OH)]−1 substitutions. Except for Tur I, which is enriched in Rb (0.02–21.9 ppm) and Cs (0.01–0.63 ppm), due to high fluid-rock reaction, the BAG tourmalines are enriched in Cr (2.0–3908 ppm), Ni (5.0–222 ppm), Co (0.05–27.4 ppm), Sr (41.8–3031 ppm), Sc (1.6–281 ppm), V (32.0–701 ppm), Al (0.01–0.35 apfu), Fe (0.59–1.47 apfu), and Mg (0.67–2.43 apfu), suggesting metasedimentary-derived components. Boron isotopes display bimodal populations from −16.8 to −12.0 ‰ (Tur II to IV) and from −23.0 to −19.0 ‰ (Tur I). We propose that greenschist to amphibolite facies metamorphism during the Pan-African Orogeny devolatilized country-rock metapelites and produced a metamorphic-hydrothermal fluid responsible for BAG tourmalinization. These metamorphic-derived Tourmalines show no relationship to the magmatic-hydrothermal fluid derived BAG gold ore. Our results show that the BAG tourmalines may not serve as indicator minerals in exploration for gold mineralization in the BAG Goldfield in northern West Nigerian Subshield.

Genetic features of tourmaline from the magnesian skarns of Kuhilal deposit, Southwestern Pamir

Background. Although the Kuhilal deposit of Mg skarns has been thoroughly studied, tourmaline in their composition has never been considered [5, 6, 16]. Uvite, a tourmaline variety, was mentioned in the list of rare minerals only with its crystallochemical formula [17]. L.N. Shabynin, a great connoisseur of skarns, noted the rarity of tourmaline in Mg skarns of various deposits. Tourmaline of this type is characterized by a strong variation in ferruginosity and a significant content of Ca. At the same time, Ca:Na varies from 2:3 to 20:1 [15]. According to N.N. Pertsev, another major specialist in skarns, tourmaline is a rare mineral of Mg skarn deposits [11]. The boron-iron ore deposit of Tayozhnoe, Aldan shield, is an exception. This deposit belongs to skarns, where a widespread development of tourmaline was noted [12]. Tourmaline (dravite) of this deposit was found only in silicate metamorphic rocks containing marbles and Mg skarns.Aim. To determine the paragenesis, crystal morphology, chemical composition, and genetic characteristics of tourmaline from Kuhilal Mg skarns.Materials and methods. Large aggregates (more than 10 cm across) and crystals of apple-green tourmaline from the Mg skarns of the Kuhilal deposit were investigated. Samples of Mg skarns containing tourmaline were collected and documented by the authors during fieldwork research at the Kuhilal deposit in 2017. The samples were examined by mineralogical and petrographic methods, X-ray diffraction analysis using a DRONE-3M (analyst A.V. Fedorov, Sergo Ordzhonikidze Russian State University for Geological Prospecting), microprobe analysis using a Cameca SX 100 in 15 kV 30 nA shooting mode (analyst V.I. Taskaev, IGEM RAS), and X-ray fluorescence analysis using an AXIOS advanced spectrometer with an X-ray tube equipped with a 3 kW Rh anode and Philips PW-2400 with a sensitivity of 10-4% (analyst A.I. Yakushev, IGEM RAS). The majority of studies were conducted at the Department of Mineralogy and Gemology of the Sergo Ordzhonikidze Russian State University for Geological Prospecting.Results. Tourmaline from the Mg skarns of the Kuhilal deposit has been studied for the first time. This mineral is rare for skarn rocks. Two tourmaline mineral associations were identified: with spinel and forsterite, and with chlorite and serpentine. In terms of chemical composition, tourmaline isattributed to fluorine-containing uvite with a close to zero ferruginosity. Uvite is characterized by a lenticular shape with a rarely observed, underdeveloped prism. Its mineralogical properties, chemical composition, and formation conditions were analyzed. Tourmaline can be used to make inexpensive jewelry inserts.Conclusion. Tourmaline crystallization occurred under the conditions of granulite facies of regional metamorphism. For boron formation, an apo-sedimentary, evaporite source is assumed.

Petrogenesis of granitic pegmatite veins: Perspectives from major element and B isotope in tourmalines, Chakabeishan, Northern Tibetan Plateau

The petrogenesis of regionally zoned granitic pegmatite veins remains debated. Because of the economic significance, we carried out a study on the Chakabeishan (CKBS) pegmatite-type Li-Be deposit, eastern North Qaidam Tectonic Belt, Northern Tibetan Plateau, by means of in-situ major element and B isotope compositions of tourmalines in the beryl-bearing and spodumene-bearing pegmatite veins. Tourmalines (Tur-Be) from the beryl-bearing pegmatite are homogeneous schorl with low Mg/(Mg + Fe), high Na/(Na + Ca) and YAl, suggesting that they are of magmatic origin. Two generations of tourmalines (Tur-Li) from the spodumene-bearing pegmatite are identified: (i) the crystal cores (mostly elbaite and Li-rich schorl with subordinate schorl) are consistent with being of magmatic origin crystallized at the magmatic stage; (ii) the crystal rims (schorl) are best understood as the overgrowth at the later hydrothermal stage. Tur-Be and Tur-Li show an obvious difference in core-to-rim B isotopic variation trend with δ11B decrease in Tur-Be and increase in Tur-Li. The core-to-rim δ11B decrease in Tur-Be results from degassing during its host pegmatitic melt evolution, whereas the core-to-rim δ11B increase in Tur-Li is related to fluid exsolution. The estimated δ11B values for the initial melts of the beryl-bearing and spodumene-bearing pegmatites are −10.46‰ and −10.78‰, respectively, indicating that they most likely originate from protracted fractional crystallization/differentiation of granitic intrusions rather than partial melting of metapelite. Both Mg/(Mg + Fe) ratios and Li abundances in the cores of Tur-Be are lower than those of Tur-Li, suggesting that Tur-Li crystallizes from chemically more evolved melts.

Tourmaline Breccias from the Río Blanco-Los Bronces Porphyry Copper District, Chile: Constraints on the Fluid Source and the Utility of Tourmaline Composition for Exploration

Abstract Tourmaline-cemented breccia bodies host much of the ore in the Río Blanco-Los Bronces porphyry Cu-Mo deposits. We determined the chemical and B isotope composition of tourmaline as well as S isotope ratios of anhydrite and sulfide minerals to shed light on the composition and origin of mineralizing fluids. Also, the utility of tourmaline as an indicator mineral was tested by comparing mineralized and barren breccias. Tourmaline in mineralized samples has a narrow Mg range (1.5–2 apfu) and variable, generally low Al contents (4–6.5 apfu). A strong negative correlation of Al with Fe indicates monovalent substitution of Al and Fe3+, implying relatively oxidizing fluids. In contrast, tourmaline from barren breccias has a narrower Al range (6–7 apfu), lower and more variable Mg (0.2–2.5 apfu), and a strong negative Mg-Fe correlation, suggesting more reduced fluids with a dominance of Fe2+. These features and the implications of redox contrast may have exploration significance. Tourmaline from all breccia samples yielded δ11B values from 1.8 to 7.9‰. A magmatic source of boron is concluded from the identical B isotope values of granite-hosted tourmaline in the district (1.2–7.7‰) and from the similar range of regional volcanic and porphyry rocks in the Central Andes. The δ34S values of coexisting anhydrite (11.6–14.5‰) and chalcopyrite (–1.5 to –0.2‰) in mineralized breccia give S isotope exchange temperatures of 377° to 437°C, consistent with fluid inclusion temperatures. Total sulfur δ34Sfluid estimates between 1.4 ± 3.9 and 8.8 ± 1.3‰ are broadly consistent with a magmatic source but not well constrained. However, published O and H isotope ratios of quartz and tourmaline from the Río Blanco-Los Bronces breccias have a clear magmatic signature, so this is the preferred scenario. Mass balance simulations of the boron budget show that typical magma flux rates, water contents, and boron concentration for the Central Andes could produce the estimated 107 tons of boron in the Río Blanco-Los Bronces breccias within the 4-m.y. duration of porphyry intrusions if (1) magma accumulated and evolved at midcrustal levels before emplacement and (2) boron partitioned strongly to the fluid phase (DBfluid/melt > 3).

Hydrothermal fluid evolution in the Cuonadong Sn–W–Be polymetallic deposit, southern Tibet: indicated by the in–situ element and boron isotope compositions of tourmaline

The Cuonadong Sn–W–Be polymetallic deposit in the Himalayan leucogranite belt is a representative hydrothermal deposit. The role of fluid exsolution directly from magma and the fluid reaction with surrounding rocks for ore-forming element enrichment is still controversial. Tourmaline is a significant B-bearing mineral in the hydrothermal deposit, and its geochemical and B isotopic signatures can record the source and evolution of the ore-forming fluid. Two types of hydrothermal tourmaline in the hydrothermal quartz vein (Tur-1) and skarn (Tur-2) were used in this study. Both Tur-1 and Tur-2 have low X-site occupancy and mainly belong to the alkali group. Tur-1 plots in the schorl field, whereas Tur-2 is largely Mg-rich dravite. The B isotope analyses of Tur-1 have δ11B values of −13.7 to −13.2‰, whereas Tur-2 has higher δ11B values of −11.1 to −9.3‰. The distinct contact relationship and geochemical compositions suggest that Tur-1 in the hydrothermal vein was formed from a magmatic-hydrothermal fluid with little influence from surrounding rocks and had a genetic relationship with the Cuonadong leucogranite, whereas Tur-2 in the skarn involved more fluid from surrounding rocks with high δ11B values and strong metasomatic texture. The higher ore-forming element contents in Tur-2 than those in Tur-1 indicate that the reaction between the magmatic exsolution fluid and the surrounding rock is essential for the enrichment and precipitation of ore-forming elements.

In-situ boron isotope and chemical composition of tourmaline in the Gyirong pegmatite, southern Tibet: Implications for petrogenesis and magma source

Leucogranitic rocks, mainly including leucogranite-pegmatite systems, have been found to be widely distributed in the South Tibetan Himalaya, and they have received considerable interest because of their significance in crustal evolution and associated rare-metal mineralization. Although the nature and geodynamic setting of the Himalayan leucogranites have been well documented by numerous studies, the pegmatites spatially associated with these leucogranites are still poorly understood. Tourmaline is a ubiquitous phase from the leucogranite to the pegmatite. We have therefore conducted in situ major and trace element and boron isotope investigations of tourmaline from the Gyirong pegmatite, synthesizing published data on the Gyirong leucogranite, to document the origin of tourmaline and its genetic implications. Two types of tourmaline (Tur-Ⅰ & Tur-Ⅱ) have been identified in this contribution and they are enriched in Fe, Si and Al but depleted in Mg and Ca, with Mg/(Mg+Fe) ratios ranging from 0.22 to 0.45. Accordingly, the tourmalines belong to the alkali group and have schorl composition. Trace elements, such as Zn, Ga, V, Sc, Li, Sn, Sr, and Co in the tourmalines are relatively enriched, whereas, other trace elements record low concentrations less than 10 ppm. The trace element concentrations of tourmaline are mainly controlled by melt composition. Morphological and geochemical characteristics reflect that the tourmalines from the Gyirong pegmatite are magmatic in origin. The Gyirong pegmatitic tourmalines have S-type granitoids and pegmatites boron isotopic signatures with a tight range of δ11B values between −11.8 and −9.7‰, which is consistent with the magmatic tourmalines (Mg-poor) of the Gyirong leucogranite. This study suggests that the Gyirong pegmatite was the product of crustal anatexis and that the crustal metapelitic rocks within the Greater Himalayan Crystalline Complex were the most likely source components.

Genesis of the Xianghualing tin-polymetallic deposit in southern Hunan, South China: Constraints from chemical and boron isotopic compositions of tourmaline

The Xianghualing skarn Sn polymetallic deposit (Nanling Range), with ca. 0.25 Mt Sn metal, 0.85 Mt Pb + Zn metal, and 22 Kt (Ta, Nb)2O5, is a Late Jurassic biotite granite-related Sn-Pb-Zn-Nb-Ta deposit. However, our understanding of how magmatic-hydrothermal processes contributed to its mineralization remains poor. Tourmaline has the capacity to retain primary elemental and isotopic information throughout the tin-related ore-formation process, allowing us to use the in-situ major, trace elemental, and boron isotopic compositions of tourmaline to research the origin of tin-bearing fluids and to trace the magmatic-hydrothermal evolution associated with tin mineralization. Five types of tourmaline are recognized based on petrography: i) late-magmatic tourmaline (Tur-I) disseminated in the biotite granites; ii) hydrothermal tourmaline (Tur-II) in massive schorl-fluorite association from the endoskarn zone; iii) hydrothermal tourmaline (Tur-III) with long columnar crystals in massive schorl-epidote association from the exoskarn zone, is related to Sn-Pb-Zn mineralization; iv) hydrothermal tourmaline (Tur-IV) from tourmaline-fluorite veins in exoskarn within the marble; and v) hydrothermal tourmaline (Tur-V) from cassiterite-sulfide-tourmaline-fluorite-topaz veins hosted by quartz sandstone, and is directly related to Sn-Pb-Zn mineralization.Tourmaline from the Xianghualing tin-polymetallic deposit dominates the alkali group and the schorl-dravite solid-solution series. Distinct Fe/(Fe + Mg) and Na/(Na + Ca) ratios of Tur-II to Tur-IV reveal different degrees of B-rich fluid-rock (granite or carbonate) interaction. Trace element variations in tourmaline are principally governed by local fluid-rock interactions and melt/fluid compositions in a low fO2 environment. Late-magmatic tourmaline (Tur-I) and hydrothermal tourmaline (Tur-V) related to vein-type Sn-Pb-Zn mineralization have similar Ca, Na, Al, Mg, Fe, and REE + Y contents, indicating that magmatic-hydrothermal exsolution controls Sn-Pb-Zn enrichment. This process occurs in a reducing environment without significant water–rock reactions, demonstrating that high fO2 or acid-consuming reactions are unnecessary for cassiterite precipitation. Variations in water–rock reactions between magmatic fluids and the carbonates cause differences in tourmaline elemental concentrations (e.g., Ca, Al, and Pb) between the magmatic and skarn phases. There are two main paths of tin mineralization in the Xianghualing deposit: (1) fluorine- and boron-rich fluid exsolution at the magmatic-hydrothermal transition stage is crucial for releasing tin and forming cassiterite during the direct cooling process; (2) fluid-rock interaction between tin-bearing magmatic fluids and carbonates facilitates the precipitation of cassiterite. The small variation in δ11B values for all types of tourmaline (-14.8 to −11.6 ‰) suggests that B-rich fluids are mainly derived from magmatic differentiation and fluid exsolution, with insignificant B isotopic fractionation. Magmatic-hydrothermal evolution at the Xianghualing Sn-polymetallic deposit can be unraveled by using the element and B-isotope fingerprints of tourmaline, which might be used as a prospecting aid for Sn-polymetallic deposits.

Tracing the evolution of the pegmatite system and its interaction with the country rocks by chemical and boron isotope compositions of tourmaline in the Qinghe pegmatite from the Chinese Altay orogen

伟晶岩及其围岩蚀变带的电气石记录了伟晶岩体系的演化、流体出溶以及流体-围岩相互作用的过程。本文对阿尔泰造山带青河伟晶岩及蚀变围岩中电气石进行了系统的化学成分和硼同位素分析，以探讨伟晶岩体系的源区性质、岩浆演化、流体出溶规模等重要科学问题。根据矿物成分和结构构造不同，伟晶岩可分为边缘带、过渡带和核部带。蚀变围岩从接触带开始向外，依次出现带状蚀变围岩、细粒蚀变围岩和粗粒蚀变围岩。伟晶岩蚀变围岩电气石的形成与伟晶岩出溶流体（提供B、P等）和围岩（提供Fe、Mg）相互作用有关。电气石主、微量元素和硼同位素分析表明，所有电气石都属于碱性电气石中的铁电气石-镁电气石固溶体序列，电气石中（R1+Al）（Na+R2）_-1、（Al+O）（R+OH）_-1、（Na+Mg）（Al+X_vac）_-1、（X_vacAl）（NaR²⁺）_-1、FeMg_-1及MnMg_-1的替代关系记录了伟晶岩岩浆逐渐富集Fe、Mn、Al，亏损Na、Mg、Ca的演化路径。结合各分带V含量和Fe³⁺-Al相关性，我们认为原始伟晶岩浆具低氧逸度的特征，而伟晶岩脉边缘带较高的氧逸度可能与伟晶岩浆与围岩相互作用有关。伟晶岩电气石具有比花岗岩电气石低得多的REE总量和（La/Yb）_N比值，表明青河伟晶岩并不是花岗岩体系分离结晶的残余岩浆，而可能是云母片岩在中地壳角闪岩相条件下低程度部分熔融的结果。蚀变围岩电气石的硼同位素组成仅稍重于未蚀变围岩的电气石，显示青河无矿化伟晶岩演化晚期流体出溶比较有限。由于伟晶岩中的电气石硼同位素组成明显轻于围岩（云母片岩），推测云母片岩并非伟晶岩的唯一源区，而可能存在一定含量的黏土岩源区。

Magma contamination in Himalayan leucogranite by metapelite host rocks: insights from chemical and boron isotopic compositions of tourmaline

ABSTRACT Contamination is a common scenario of intracrustal magmatic processes that may significantly change the compositions of involved anatectic melts. Here, we present major element and B isotope analyses on tourmalines from the Guowu leucogranite and their host rocks, which allow us to unravel the potential contaminations of the Himalayan leucogranites by metapelite host rocks. Three types of tourmaline have been identified in our samples. Tur-M in the host schist has the highest Mg/(Mg+Fe) ratio (0.52–0.62) and low Al content, suggesting its metamorphic character. Tourmalines in the leucogranite (Tur-L) and in the contact zone between leucogranite and schist (Tur-C) have an identical core-rim texture, and their cores are characterized by the lowest Mg/(Mg+Fe) ratio (0.13–0.18) and high Al content, which are consistent with the composition of magmatic tourmaline in the Himalayan leucogranites. The rims of Tur-L show higher Mg/(Mg + Fe) ratio (0.32–0.45) and moderate Ca, Ti, F contents, reflecting the contribution of the host schist via contamination. The composition of Tur-C rims is similar to that of Tur-M, suggesting a more significant contribution from contaminated schist than that for Tur-L. The different types of tourmaline share consistent B isotope compositions with δ11B ranging from −14 to −12‰. The compositional characteristics of the contaminated tourmaline and the mineral assemblage in the contaminated zone suggest that the involved components derived from the host schist, probably produced by the decomposition of tourmaline, biotite, plagioclase, primarily include Ca, K, Mg, Ti, B and F. In addition, contamination by tourmaline-bearing metapelite may be an alternative interpretation responsible for the peraluminous character of the Himalayan leucogranites. The occurrence of contaminated tourmaline in leucogranites suggests that contamination of host rocks is a possible way promoting the formation of tourmaline in the Himalayan leucogranites, and tourmaline is useful for deciphering the related contamination processes.

Chemical and boron isotopic compositions of tourmalines from the Lianyunshan Nb-Ta pegmatite in northeastern Hunan, China: Insights into fluid and metallogenic sources

Although tourmaline is common in pegmatite-related Nb-Ta deposits and metasedimentary cover rocks in the Lianyunshan Complex of northeastern Hunan Province, South China, its fluid sources and genetic relationship to the host pegmatite remain unresolved. In this contribution, we chose the less-studied tourmaline minerals and analyzed their chemical (major, trace, and rare earth elements) and boron isotopic compositions to shed light on the process of pegmatite-related Nb-Ta mineralization. Lianyunshan tourmalines are both metasedimentary-hosted (Tourmalines I and II) and pegmatite-hosted (Tourmaline III), belong to the alkali group, have mainly schorlitic compositions, and contain dominantly hydroxyl anionic species. Element substitution is controlled by the activities MgFe-1, alkali-deficiency vector R1Al[Na(FeMg)]-1, AlO[R(OH)]-1, and □Al(NaR)-1. Lianyunshan tourmalines define a trend consistent with foitite to magnesio-foitite compositions. Lianyunshan tourmalines are poor in Fe3+, with a Fe3+/Fe2+ ratio of zero in all samples. This feature, as well as their non-alignment with the FeAl-1 vector, suggests precipitation from a reduced fluid. Fluid salinity may have been moderate to low given the low Na contents of all samples (0.60–0.66 apfu). Tourmaline I is rich in Mg and Ti, but the contents of these elements decrease in Tourmalines II and III. Fluid fractionation may have played a role in the decreasing Mg and Ti contents with corresponding enrichment in the contents of Fe, Na and X□ as Tourmalines II and III precipitated. Unusual enrichment of Li is found in the pegmatite-hosted Tourmaline III (mean 97 ppm) relative to Tourmalines I and II (means ∼ 31–33 ppm), and of Sn in Tourmaline II (mean 291 ppm) relative to Tourmalines I and III (means ∼ 3–10 ppm). δ11B values are similar for Tourmalines I (–14.21 to –12.09 ‰), II (–15.07 to –13.71 ‰), and III (–14.48 to –13.55 ‰), being consistent with the δ11B of typical S-type granites (∼−16 to −8 ‰) such as the S-type muscovite monzogranite host of the Lianyunshan tourmalines. We conclude that the Lianyunshan tourmalines precipitated from an evolved melt that formed muscovite monzogranite.

Elemental and boron isotopic variations in tourmaline in two-mica granite from the Cuona area, Tibet: Insights into the evolution of leucogranitic melt

Two-mica granite is the most common magmatic rock type in the Himalayan leucogranite belt, which has close relationship with rare metal mineralization. Its genesis is generally attributed to magmatic differentiation. In recent years, the mineral geochemical compositions are increasingly used to study magmatic differentiation, which are significant for deciphering the melt evolution and element migration processes. In this study, in-situ major and trace element and boron isotope compositions for tourmalines from two-mica granites in the Cuona and Cuonadong leucogranites in the Cuona area are conducted to determine microscopic changes in mineral assemblages and geochemical compositions. Analytical results show that the tourmalines in the Cuonadong leucogranite were crystallized earlier relative to the tourmalines in the Cuona leucogranite during magmatic differentiation. The volatile contents have a genetic relationship with incompatible elements in tourmaline, which is possibly responsible for the formation of tourmaline zonation and the enrichment of Sr, Zn, and Pb during magmatic differentiation. The B isotopic composition of tourmaline in the Cuona area suggests that the granitic magma was dominantly derived from the partial melting of the metasedimentary source rocks. Their B isotope variations likely resulted from fluid exsolution during B-rich melt evolution. High rare metal contents in tourmalines indicate that the two-mica granites in the Cuona area may have great mineralization potential.

Tourmaline chemistry, boron, and strontium isotope systematics trace multiple melt–fluid–rock interaction stages in deeply subducted continental crust

The generation, transport, and recrystallization of slab-derived melts/fluids play a critical role in the deep recycling of elements in subduction zones. While boron (B) isotope systematics have been invoked as an important tracer of these processes, its behavior during metamorphic dehydration and partial melting of deeply subducted continental slabs, and the partitioning of B isotopes between minerals and melts/fluids is not fully understood. Here, we investigate these processes through an in situ study of the major, trace-element, and B-Sr isotope variations in different occurrences of tourmaline in migmatite from the Yuka terrane, North Qaidam orogen (China), which resulted from partial melting of a continental slab at different stages during subduction and exhumation. Based on textural and detailed high-resolution X-ray mapping studies, tourmaline was classified into four paragenetic generations (Tur-I, Tur-II, Tur-III, and Tur-IV). Dravitic Tur-I occurs in melanosomes and shows increasing Fe, Ca, and Ti contents from the core to the outer rim. In addition, it has relatively homogeneous Sr isotope values (0.7407–0.7416) and decreasing δ11B values (-3.8 to −8.6 ‰) and XMg (Mg/(Mg + Fe)) ratios, indicating formation in a rock buffered by an aqueous fluid during the prograde to peak metamorphism. Schorlitic Tur-II occurs within selvage zones between melanosomes and leucosomes, and yields high-Fe values and low δ11B (-13.5 to −10.9 ‰) and more variable 87Sr/86Sr (0.7343–0.7418) values, indicating crystallization in the presence of a hydrous melt external derived from breakdown of Fe-rich mineral(s) during partial melting of the subducted slab. Dravitic Tur-III formed in the matrix and also enveloped Tur-II. It shows homogeneous 87Sr/86Sr values (0.7411–0.7420) and decreasing δ11B values (-6.8 to −9.9 ‰) and XMg ratios as well as increasing Fe and Ti contents from core to outer rim. Formation of Tur-III reflects a transitional stage from hydrous melt to aqueous fluid during exhumation. Tur-IV in the leucosomes is essentially a schorl-dravite solid solution with small amounts of Ca. Its 87Sr/86Sr values (0.7402–0.7416) and δ11B values (-11.4 to −8.5 ‰) are intermediate between the respective values of Tur-II and Tur-III. The formation of Tur-IV likely results from interaction between melt and fluid and, based on its chronological sequence, and is interpreted to have formed during the exhumation stage of the Yuka terrane.Overall, the variable XMg ratios and δ11B values in the different generations of tourmalines are a consequence of the evolution of the melt/fluid at different depths within the deeply subducted slab. Decreasing δ11B values from Tur-I to Tur-II and Tur-III are controlled by the breakdown of different minerals during partial melting or metamorphic dehydration of the subducted slab, while the co-variations of the elemental geochemistry and B isotopic compositions of tourmaline reflect different depths of formation during subduction and exhumation of the lithosphere. These observations suggest tourmaline may serve as a useful tracer of multiple melt/fluid–rock interactions and of boron cycling in continental subduction zones. The heterogeneity of δ11B in melts/fluids at different depth levels of the continental subducted slab may also result in locally variable B isotope values in syn- and post-collisional magmas.

Development of Tourmaline-Bearing Lithologies of the Peraluminous Tusaquillas Composite Granitic Batholith, NW Argentina: Evidence from Quartz and Tourmaline

ABSTRACT Textural and chemical characteristics of quartz and tourmaline found in tourmaline-rich orbicules, greisens, pegmatites, and tourmalinite segregations associated with the peraluminous leucogranitic Tusaquillas Batholith Complex of northwest Argentina exhibit both magmatic and hydrothermal features. Imaging of quartz by optical cathodoluminescence and scanning electron microscopy cathodoluminescence shows three stages of development. Stage-1 quartz, considered magmatic, develops as large grains in pegmatites that have optical cathodoluminescence homogeneity; as anhedral relict grains partially replaced by stage-2 hydrothermal quartz in tourmalinite segregations, orbicules, and greisens; and as idiomorphic grains with irregularly spaced oscillatory zoning seen in scanning electron microscopy cathodoluminescence in orbicules. Stage-2 quartz, interpreted as hydrothermal, partially replaces stage-1 quartz and generation-1 tourmaline in most lithologies. Stage-3 quartz, a late hydrothermal stage, occurs in all lithologies as weakly luminescing quartz in healed quartz fractures with abundant fluid inclusions, commonly associated with the crystallization of irregular late-stage tourmaline. Multiple generations of tourmaline span magmatic to hydrothermal phases of development. In all lithologies, generation-1 tourmaline is compositionally similar: highly aluminous (range of average values of Altotal = 6.31–6.95 apfu), markedly Fe- and X□-rich (XMg = 0.01–0.17, X□= 0.21–0.51), and having variable F and WO (F = 0.00–0.57 apfu, WO = 0.00–0.40). Generation-1 tourmaline is interpreted as magmatic with compositions reflecting the chemical environment of the host lithologies and with compositional zoning patterns characteristic of both closed- and open-system behavior, possibly related to the transition to subsolidus conditions. Similar to generation-1 tourmaline, later-stage generations-2 and -3 tourmaline compositions are highly aluminous (range of average values of Altotal = 6.38–6.79 apfu), markedly Fe- and X□-rich (XMg = 0.00–0.20, X□= 0.28–0.40), and variably F- and WO-enriched (F = 0.07–0.57 apfu, WO = 0.00–0.31), but notably poorer in Ca and Ti (<0.01 apfu). The later-stage tourmaline is considered to have developed during the subsolidus hydrothermal conditions. External chemical contributions to tourmaline compositions from the country rocks appear to be minor to nonexistent. The X-site and W-site occupancies of the late-generation tourmaline implies subsolidus invasive alkaline, saline aqueous fluids with high Na but minimal Ca contents derived from the crystallizing leucogranites and related rocks across the solidus-to-subsolidus transition.

Tourmaline as a recorder of the magmatic-hydrothermal evolution in the formation of pegmatite: In-situ elemental and boron isotopic compositions of tourmaline from the Qinghe pegmatite, Chinese Altay orogen

• Tourmalines record the magma-hydrothermal evolution of pegmatite-forming magma . • Extremely low REEs of tourmaline indicate direct melting of metapelite to form pegmatite . • Boron isotopic variation reflects the scale of fluid exsolution during pegmatitic evolution. We conducted elemental and boron isotope studies on tourmalines from a single pegmatite dyke of the Altay orogen to investigate its origin and evolution. The pegmatite dyke is composed of border, intermediate and core zones. The border and intermediate zone tourmalines mostly belong to schorl, while the core zone and magmatic hydrothermal tourmalines to dravite. Most tourmalines follow the (Na + Mg) (Al + X vac ) -1 , FeMg -1 and MnMg -1 exchange vectors. The core zone tourmalines show positively correlated FeO t and MgO, and negatively correlated FeO t and Al 2 O 3 , suggesting a rise of f O2 in this zone. Moreover, they show higher B, Mg, Sr and Eu than other tourmalines due to magmatic evolution. Tourmaline from the Altay pegmatite is characterized by extremely low REE abundances (<3 ppm) when compared with those from granites, suggesting an origin from direct melting of metapelitic rocks. The border and intermediate zone tourmalines have comparable boron isotopic compositions with δ 11 B = −14.0 to −12.2‰, which are similar to the mica schist (−13.2 ‰ to −12.2‰), but slightly heavier than the core zone and magmatic hydrothermal tourmalines (−14.4‰ to −13.3‰). This suggests derivation of the pegmatite from melting of the metapelite without causing boron isotope fractionation, whereas late-stage fluid exsolution caused somewhat boron fractionation. This may be indicative of only small amounts of fluids being released from the barren pegmatite-forming magma. Tourmalines from mineralized pegmatites basically exhibit heavier boron isotope compositions than those from barren pegmatites, due probably to more involvement of evaporate in the source of the former.

Fluid sources and mineralizing processes in greenstone belts: a stable isotope (O, H) comparison between the weakly mineralized Moly-Desgagné–Guercheville system and Val-d’Or orogenic gold deposits, Canada

In orogenic gold systems, the source of the fluids and the processes leading to mobilization, transport, and deposition of gold remain debated. Most studies focus on endowed rather than on gold-poor orogenic systems to unravel the “key” parameters of gold mineralizing processes. Here, we present stable isotope (O, H) data from the Moly-Desgagné–Guercheville fault system (gold-free to low gold endowment) in the Chibougamau area, Abitibi greenstone belt, Canada. The gold-free Moly-Desgagné showing and nearby poorly endowed Hazeur showing and Monster Lake deposit share similar features typical of orogenic systems. The stable isotope compositions of tourmaline and quartz from the Moly-Desgagné–Guercheville fault system also display similar characteristics, such as (1) temperature of vein formation of 345 ± 86 °C (1σ); (2) fluid mixing between an upper crustal reservoir (low T – high δD – low δ18O) and a metamorphic water reservoir (high T – low δD – high δ18O); and (3) positive δDfluid values consistent with multiple boiling-condensation cycles related to fault-valve processes. These characteristics are similar to those from the gold-endowed Val-d’Or vein field. The difference in gold endowment between the Moly-Desgagné–Guercheville and Val-d’Or vein fields may be related to the nature of the fault system (i.e., terrane-bounding or “intragreenstone belt”) and the volume and (or) composition (i.e., gold, its ligands, CO2) of the fluid source rocks.

Tourmaline growth in pelitic schist and quartzite: A textural, chemical and B-isotopic study from the Gangpur Schist Belt, eastern India

Tourmaline is the principal repository of boron in crustal rocks and therefore useful for tracing B-cycling during prograde dehydration and retrogression of supracrustal rocks. Here, we use the major-trace element, and B isotope composition of tourmaline from schists, quartzites, and tourmaline-quartz veins of the Gangpur Schist Belt in eastern India to constrain the source of boron and the physicochemical evolution of B-rich fluids during prograde dehydration metamorphism. Tourmaline growth and re-equilibration in rocks of the Gangpur Schist Belt was a multi-stage process involving several fluid sources. The δ11B varies between −6‰ and −18‰, indicating a dominantly continental source for boron. Tourmaline in schists, quartzites, and tourmaline-quartz veins grew over a wide range of P-T conditions and record multiple episodes of metamorphic dehydration between ca. 1.6 Ga and ca. 0.95Ga. The tourmaline in tourmaline-quartz veins and quartzites has lighter B-isotope composition, typical of continental detritus, while those in the schists and quartzites record pelite-dehydration signature with values decreasing gradually from ca. −12‰ in the cores to ca. −17‰ in the rims. Heavier isotopic compositions (δ11B of ca. −6‰) measured in some grains in the pelites and quartzites indicate boron contribution from meta‑carbonate sources. The mixing of a heavier B-rich metacarbonate-derived fluid with pelite-derived metamorphic fluids could explain the lower B-isotope values in such tourmaline. The study also attempts to constrain the controls on the intake of trace elements in tourmaline. The results suggest that the partitioning of Mn, Y, V, Co and Ti in tourmaline is affected by the growth of porphyroblast phases such as garnet, staurolite, and biotite, while Li, Sr, Zn and Sn reflect the signature of the metamorphic fluid.

Magmatic-hydrothermal processes of the Laojunshan metamorphic massif in Southeastern Asia: Evidence from chemical and B-isotopic variations of deformed tourmalines

Multi-stage fluid influx has led to the volumetrically important occurrence of granite-related tourmalines at the Laojunshan metamorphic massif in Southeast Asia, which exhibits variable deformed structures and decomposition textures. These include disseminated tourmaline porphyroclasts (type-I), crosscutting tourmaline-quartz veins (type-II) and tourmaline veinlets (type-III). The chemical variations of these three types of tourmalines reveal a source transition from early boron-rich melt to late hydrothermal fluid, accompanying with variable fluid-rock interaction. Hydrothermal tourmalines are characterized by higher Mg/(Mg + Fe) ratios, lower Na/(Na + Ca) ratios, more pronounced positive Eu anomalies, higher Sr and heavy rare earth element (HREE) concentrations, and lower Li, Nb, Zr, Hf, and light rare earth element (LREE) concentrations compared to magmatic tourmalines. The tourmalines exhibit δ11B values ranging from −13 to −7.9‰ for type-I, from −15.5 to −7.5‰ for type-II, and from −18.6 to −11.6‰ for type-III. This variation is mainly due to multiple fluid exsolution, Rayleigh fractionation and the mixing of two isotopically distinct sources. UPb dating results of magmatic zircons from two tourmaline-bearing gneisses reveal the ages of magma emplacement and crystallization from 445 to 420 Ma. Magmatic-hydrothermal fluids released from Silurian peraluminous granites are responsible for the formation of the disseminated type-I tourmalines. Further hydraulic fracturing during exhumation led to the formation of the tourmaline-quartz veins (type-II) and the tourmaline veinlets (type-III), possibly from Cretaceous granitic magma-derived fluids that interacted with metamorphic rocks. The decreasing grain size resulted from decomposition and cataclasis coupled with the overpressured fluids weakening the host rocks along fractures and eventually promoting the deformation during exhumation of the metamorphic massif. The chemical and isotopic compositions of tourmaline suggest that the oxidized conditions possibly through fluid boiling and fluid-rock interaction may promote the regional Sn-mineralization.

Tourmaline boron isotopes trace metasomatism by serpentinite-derived fluid in continental subduction zone

Subduction of low-density continental crust to subarc depths is generally associated with preceding subduction of the oceanic slab. However, the original oceanic signature is often overprinted by subsequent fluid metasomatism from the subducting continental crust. As a result, it is generally difficult to trace the geochemical processes of previous oceanic subduction in collisional orogens. This issue can be potentially resolved by applying B isotopes to metamorphic rocks from continental subduction zones. Here a combined study of whole-rock geochemistry and in situ tourmaline B isotopes was carried out for coesite-bearing whiteschist and phengite schist, as well as country rock metagranitoids from the Dora-Maira Massif in the Western Alps. While all these metamorphic rocks have a similar protolith of Permian granites, whiteschist and phengite schist experienced Mg-rich fluid metasomatism during the continental subduction, evidenced by their much higher MgO contents than the metagranitoids. Tourmaline in the metagranitoid (Tur-G) is schorlitic (XMg = 15–55) with low δ11B values from −13 to −6‰, and is consistent with a magmatic origin. In contrast, tourmaline in metasomatic rocks (Tur-S) is mainly dravitic with the highest XMg worldwide (XMg = 90–98) and high δ11B values of −5 to +1‰. Integrated with whole-rock geochemistry and previous studies, Tur-S is interpreted to grow during the infiltration of external fluids that were highly enriched in MgO and relatively enriched in 11B. According to the B isotope compositions of precursor tourmalines, it is estimated that the external fluids had significantly higher δ11B values than +2.4‰. Based on B isotope compositions of both Tur-S and metasomatic fluids, our quantitative modeling suggests that the metasomatic fluids most likely originated from the mantle wedge serpentinite that was formed during the preceding oceanic subduction stage. Therefore, tourmaline B isotopes in the ultrahigh pressure metamorphic continental crust can be used to trace preceding fluid metasomatism at the interface between oceanic slab and mantle wedge. The mantle wedge serpentinite plays an important role in modifying the geochemical composition of deeply subducted supracrustal rocks and probably also the mantle sources of arc magmas.

Chemical and boron isotopic composition of tourmaline from the Gonghe Triassic intermediate-acid intrusive rocks, Qinghai and its implications for evolution of the magmatic-hydrothermal system

青海共和盆地干热岩经历的热历史过程、热源是了解干热岩地热藏形成亟待解决的难题, 不同时期、不同类型的脉体可为其经历的热过程、热源提供证据。经调查发现, 盆地东北当家寺岩体及井下干热岩中电气石脉体与该区后期断裂产状相近, 是否代表后期热事件需要确定。本研究选择对GR1井、DR3井中酸性侵入岩岩芯和当家寺露头岩体中发现的电气石脉体开展了岩相学、锆石年代学、电子探针、LA-MC-ICPMS原位微量元素及B同位素分析, 以约束电气石脉体的成因和源区。结果表明, GR1、DR3井岩芯及当家寺岩体含有电气石脉体的岩性分别是碱长花岗岩、高镁闪长岩及二长花岗岩; 其中露头区花岗岩体中电气石脉的宽度约20cm, 产状直立, 其围岩的形成时代为239~241Ma。背散射及显微图像特征揭示, GR1井和DR3井下中酸性侵入岩及当家寺岩体中电气石为碱族的黑电气石和镁电气石, 具有远近不同的多个流体来源。<i>δ</i>¹¹B分布在-11.50‰~-11.93‰, 与大陆地壳平均的同位素组成<i>δ</i>¹¹B值(-10‰±3‰)相近。结合区域地质资料, 认为在晚三叠世时期, 该区域整体处于碰撞期或后碰撞期, 陆壳加厚发生部分熔融形成S型花岗岩(~220Ma), 其中含硼的热液流体侵位于早期具有俯冲背景的I型花岗岩(~240Ma)中形成电气石脉。